Data Validation formula breaks for half of incorrect data

Hi there,
I'm having an issue using the following formula in my spreadsheet:

=IF(--ISBLANK(D6)=1,IF(--ISBLANK(J6)=1,1,0),IF(--ISBLANK(J6)=1,1,IF(--OR(J6="P",J6="F")=1,1,0)))

It's designed to first check that a preceding cell (D6) is blank, and if it is, nothing should be allowed to be typed in said cell (J6). If (D6) is not blank, then (J6) can either be P or F for Pass or Fail. However, it only works half of the time; when (D6) is filled, it will only allow P or F. But when (D6) is not filled, it will allow me to enter anything in (J6) even though the logic statement returns 0 when I do. Am I misunderstanding how the custom data validation performs its check for fail or pass, or is there something else I should be doing, or what? I've attached a sample spread with a basic example of my issue for anyone to try, and hopefully fix.

Thanks!